A Portland police officer keeps watch at the corner of High Street and Congress Street Monday. High Street to Deering Street was closed after part of a brick facade separated from the building at 142 High Street. Buy Photo

PORTLAND, Maine — Work crews welded steel braces onto the side of the State Theatre building Monday afternoon after the aging facade came undone, raining brick and debris onto the street below on Sunday. The cause has not been determined.

Representatives of Stone Coast Properties LLC, the company that manages the building, did not return a call for comment. Stone Coast’s Kerryann McQuade told Portland ABC television affiliate WMTW that engineers were still evaluating the building’s breakdown Monday.

Work crews are attempting to determine if other sections are in danger of crumbling, said Portland city spokeswoman Nicole Clegg.

Despite recent events, city officials have declared the 1928 building, also known as the Congress Street Building, structurally sound.

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“The questions they’re trying to answer are: What may have caused this? Is this something that could happen elsewhere in the facade? How can this be secured in the short-term? And what are the long-term solutions for repairs?” Clegg said.

The evaluation and short-term repair work were expected to keep a roadblock on High Street, a high-volume throughway which connects commuters from the Casco Bay Bridge to Interstate 295, closed to traffic through 7 p.m. Monday.

Clegg said the city hopes to reopen one lane of traffic overnight Monday and into the Tuesday morning commute, and then reassess the situation.

The building stands on the corner of High and Congress streets. Traffic is blocked between Congress and Deering streets.

A police officer reportedly first saw the brick facade bowing from a western-facing section of the fifth and sixth floors of the building — across from Congress Square Plaza — on Sunday morning. There were no injuries reported from the incident and, in part because extra police and city workers were on staff for the Old Port Festival, the area was quickly blocked off from the public, Clegg said.

Deirdre Kelly, a film developer who rents space in the building, said the brick exteriors of the structure were repointed “a couple of years ago.”

Kelly can still access her space through a Congress Street entrance, but said she usually parks on High Street where the work crews are blocking access. The falling bricks caused confusion among people in the area, she said, who attributed the incident to everything from a sinkhole to an explosion.

“It’s a good thing nobody was underneath,” she said while passing by on Congress Street on Monday. “I was surprised yesterday. You hear all this stuff.”

The 29,000-square-foot Congress Street Building has an assessed value of nearly $3.2 million — $4.1 million including the .67 acres it sits on.

The buckling of the brick exterior on the High Street side of the building did not cancel Sunday night’s sold-out concert by Edward Sharpe and the Magnetic Zeroes at the State Theatre, venue general manager Lauren Wayne said.

The theater is separated from the rest of the edifice by a firewall and is “kind of a building inside a building,” said Wayne.